(1) Field of the Invention
The present invention relates to a nonlinear optical material, especially an organic nonlinear optical material. Furthermore, the present invention relates to an optical film or coating or optical dispersion composed of an organic nonlinear optical material and a process for the production thereof, and to an element or the like fabricated by using this organic nonlinear optical material. The organic nonlinear optical material or the like according to the present invention has excellent nonlinear optical characteristics, and therefore, can be used as a material for an electro-optical element, an optical-to-optical element and the like. Accordingly, the organic nonlinear optical material of the present invention can be advantageously utilized in the fields of the optical communication and computers.
(2) Description of the Related Art
As is well-known, a nonlinear optical material shows a second-order or third-order nonlinear optical effect under the application of a voltage or in a strong electric field of laser beams, and from the phenomen aspect, this material exerts many element functions such as optical frequency conversion, optical switching and optical amplification. Accordingly, the nonlinear optical material has attracted attention as a basic material in the fields of the optical communication and computers. Typical nonlinear optical materials heretofore developed are of the inorganic type, and crystals of KD.sub.2 PO.sub.4 (KDP), LiNbO.sub.3, KNbO.sub.3 and LiTaO.sub.3 are known. Since about 1983, development and research have been vigorously made into organic nonlinear optical materials, because it was found that organic nonlinear optical materials have superior characteristics to inorganic nonlinear optical materials. More specifically, for example, many organic nonlinear optical materials have a quadratic or second-order nonlinear optical constant more than 100 times as large as that of known inorganic crystals such as LiNbO.sub.3 on the molecule level. Some of typical organic nonlinear optical materials are described below: ##STR1##
These organic nonlinear optical materials are introduced in detail, for example, in G. E. Lipscomb, A. F. Garito and R. S. Narang, J. Chem. Phys., Vol. 75, No. 3, 1509-1516 (1981) and Kanti Jain, "Organic Materials of Optical Second Harmonic Generator", ACS Symposium Series, 233, 57-80 (1983).
Although the above-mentioned organic materials have a larger nonlinear optical effect than the inorganic materials, there is still room for further improvements. For example, organic nonlinear optical materials fail to exert a nonlinear optical effect much higher than that of LiNbO.sub.3 which is a typical inorganic electro-optical material. Polydiacetylene which is a cubic or third-order nonlinear optical material and is a polymer single crystal, as is well-known, that can be directly obtained from a monomer single crystal, can show a cubic nonlinear optical constant .chi..sup.(3) of 10.sup.-9 to 10.sup.-10, as shown in, for example, C. Sauteret, J. P. Hermann, R. Frey, F. Pradere, J. Ducing, R. H. Baughman and R. R. Chance, Phys. Rev. Lett., Vol. 36, No. 16, 956-959 (1976), but with this value, it is difficult to realize an optical-to-optical element using a laser diode (LD) as the light source. Under this background, development of an organic nonlinear optical material having larger quadratic and cubic nonlinear optical effects is desired.
Recently, an intensive study of the super lattice structure was made in the field of compound semiconductors, and the phenomenon of an increase of the nonlinear optical effect was found. The presently available super lattice is in the form of a laminate of faces, and although attempts have been made to expand this laminate to a quantum wire or quantum box and to search for new physical characteristics, these attempts have met with little success.
A material having a one-directional or one-dimensional conjugate chain, such as polydiacetylene, can be regarded as a quantum wire. Accordingly, it is expected that, if a quantum well is formed in this system, a quantum wire super lattice will be constructed and various physical properties represented by the nonlinear optical effect will be improved. Namely, the development of an organic super lattice, i.e., an organic nonlinear optical material having a quantum well structure, is desired.
It is known that the cubic nonlinear optical constant .chi..sup.(3) can be improved in a nonlinear optical film of a conjugate polymer in which a bound exciton is formed, i.e., a conjugate polymer in which a free exciton is converted to a trap exciton (see Fujitsu, Japanese Unexamined Patent Publication (Kokai) No. 62-270928. According to the technique of this patent publication, by utilizing the fact that the optical absorption spectrum is sharpened and the oscillator strength is increased if a free exciton is converted to a trap exciton, the nonlinear optical characteristics of a conjugate polymer, especially a one-dimensional conjugate polymer chain, are dramatically improved. According to this technique, since defects or impurities in the material are used as the binding centers, it is very difficult to introduce these defects into the material under artificial control. Therefore, it is desired that an improved organic nonlinear optical material having an exciton-trap center be provided.
Elements have been practically fabricated by using a polydiacetylene type long-chain conjugate molecule having a relatively large nonlinear optical effect. To fabricate such elements, however, it is generally necessary to add this long-chain conjugate molecule to a polymer such as poly(methyl methacrylate) (PMMA), and carry out a polling treatment for the formation of a film. In this dispersion system (the dispersion of the long-chain conjugate molecule in the polymer as the dispersion medium), the concentration of the added long-chain conjugate molecule, i.e., the nonlinear optical molecule, cannot be increased, and thus the nonlinear optical effect is reduced and 100% orientation of the long-chain conjugate molecule is difficult. Accordingly, the development of an organic nonlinear optical material, especially an organic nonlinear optical film, having a high concentration and a high orientation, is also desired.